Many pipelines for transport of oil and gas are not directly accessible for inspection because they are buried (on shore) or submerged (off shore). Several tools are available for In-Line-Inspection (ILI) of pipelines with a diameter of e.g. 16″, 24″ or larger. Also for smaller diameter pipes tools are available, like 4″, 8″ or 12″, but these have limited capabilities. These tools perform ultrasonic, magnetic or eddy current examination of a pipe from the inside and are often referred to as ‘pigs’. The collected data is used for assessment of the integrity and fitness for service of the pipeline.
In general the following criteria identify pipes which are difficult to inspect, especially in combination:                small diameter (less than 200 mm diameter);        multiple diameter;        small radius bends (R=1D);        back-to-back bends;        T-junctions (in an ongoing direction);        bore restrictions (weld roots, reducers, dents, valves);        single access only (bi-directional movement of inspection tool required);        short lengths, which make it difficult to control speed of inspection tool; and/or        low pressure or low flow.        
Obviously, smaller diameter pipes are more difficult to inspect from the inside because the inspection tool, including sensors, electronics and mechanics, must fit inside the pipe diameter. As stated above such tools are mainly available for larger diameter pipes like 16″, 24″ or 48″ diameter or have limitations for the smaller diameters.
A pipe bend is classified according to the centerline radius (CLR) of the bend as a ratio to the nominal pipe diameter. For example an 8″ N.P.S. pipe that is bent on a 12″ CLR is classified as a 1.5D bend or 1.5 times the nominal pipe diameter (D). The abbreviation N.P.S. denotes Nominal Pipe Size, which is a North American set of standard sizes for pipes. It should be noted in this regard that the curvature of the CLR is the same for all 8″ 1.5D bends, independent of the wall thickness or schedule of the pipe. Pipe schedules are defined in ASME B36.10 as predetermined relationships between pipe diameter and wall thickness. Depending on the situation the radius of curvature (R) of the bends can be large (only slightly bend), or 5D, 3D, 2D, 1.5D and even 1D (also named short radius bend).
In addition to the sharp curvature of bends, like in R=1D bends, also variations in diameters can exist. As stated above the relationship between diameters and wall thicknesses of pipes are standardized in schedules, depending on the required throughput and pressure. Originally, the outside diameter (OD) was selected so that a pipe, with a standard OD and a wall thickness suitable for a certain pressure, would have an inside diameter (ID) approximately equal to the nominal schedule size. Over time material and production technologies improved so it became possible to use thinner walls for the same pressure. In response to these developments the outside diameter of a particular pipe schedule was kept the same (to be able to fit new pipes to older existing pipe), but the inside diameter increased and is not anymore directly related to the pipe schedule. As a result not only the wall thickness depends on the working pressure but so does now the inner pipe diameter.
When the inner pipe diameter is accurately known then the tool can be adapted to that specific diameter before its introduction into the pipe, although it requires more preparation (time, components) to adapt the tool to an exact diameter. However, the exact inner pipe diameter may not always be known accurately, for example for older installations. In addition, if only a single pipe schedule is used for a complete installation and this pipe schedule is known then still variations in inner pipe diameter can occur because of manufacturing tolerances on the wall thickness (typically up to ±12.5%) and the outer diameter (typically ±1%). Taking these tolerances into account, the inner diameter of a 6″ pipe can for example vary from 116 mm (schedule XXS) to 157 mm (schedule 40). The inner diameter of an 8″ pipe can for example vary from 166 mm (schedule XXS) to 207 mm (schedule 40). It will be clear that the deviations allowed by the pipe schedule on the internal diameter have a stronger impact on smaller diameter sizes. Also, the inner pipe diameter may vary over the entire installation, resulting from variations in schedule due to different design pressures for different parts of the installation.
A significant part of pipeline failures (possibly more than 50%) are related to difficult to inspect pipe segments with available pigging tools, hence these pipelines are often referred to as ‘difficult to pig pipelines’ or even ‘unpiggable pipelines’. New technologies are required to deliver measurement data of such pipes to be able to perform assessment and maintenance planning. The examination may include a high resolution ultrasonic pipe wall survey, for determining the thickness of the pipe wall or detection and sizing of cracks in the pipe wall and/or determining deformations (dents), followed by engineering evaluation of the data and recommendations regarding the continued operation of the pipeline segment. Depending on the purpose of the survey the coverage may include straight pipe segments, welds between pipe segments or other parts of the pipe.
To overcome these issues it is preferred that the tool is capable of covering a certain range of pipe inner diameters without modification of the tool. Hence a free-floating in-line wall-thickness or crack detection inspection system for small diameter, unpiggable pipelines is required, which provides full coverage and high sensitivity (detection of ‘small’ defects) in small diameter pipes having sharp bends (R=1D, over 180°). Often such installations have barred and unbarred off-takes (T's, branch). The tool must be capable of operating in both directions (mechanically and measurement) with high resolution at a high speed, for example 1 m/s. Data storage capacity must be suitable for pipe lengths of several kilometers.